Answer:
The length you measured as an observer in the moving train will be smaller than the measurements of either A or B
Step-by-step explanation:
This is a phenomenon regarded as length contraction and it is one of the consequences of Lorentz transformation. This is usually felt when we are operating in a speed closer or equal to the speed of light.
The length of any object in a moving frame will smaller in the direction of motion, or contracted. The amount of contraction can be determined from the Lorentz transformation. The length is maximum in the frame in which the object is at rest.
As it is given in the attachment, the observer A will be in the fixed frame and will experience no contraction in length while you will be in the moving frame and experience length contraction.